Douglas Harper's Etymology Dictionary
ur-
prefix meaning "original, earliest, primitive," from German ur- "out of, original," from Proto-Germanic *uz- "out," from PIE *ud- "up, out" (see out (adv.)) At first only in words borrowed from German (such as ursprache "hypothetical primitive language"); since mid-20c. a living prefix in English. Compare also Urschleim under protoplasm and Urquell under Pilsner.
Wiktionary
ur-
pre. Forming words with the sense of ‘proto-, primitive, original’.